(a) Field of Invention
The invention relates to method steps for determining the time between TO and FRO received waveforms from the time reference scanning beam (TRSB) of a microwave landing system (MLS), and to apparatus for carrying out the method steps. More specifically, the invention relates to such method steps wherein the time interval between the TO and FRO received waveforms is determined by correlating the received waveforms, with apriori waveforms representative of the TO and FRO waveforms, respectively, and to apparatus for carrying out the method steps.
(b) Description of Prior Art
Known in the art are landing guidance systems wherein angular position information is derived from the time difference in received amplitude only signals. These received signals are created by a ground transmitting station which transmits a narrow beam of radio-frequency energy in a swept (or step) fashion and at a fixed angular rate. An example of such a landing system is the TRSB-MLS system. The back and forth sweeps are referred to as TO and FRO sweeps.
These are five known processes or processors, which are discussed in the current literature on MLS processing. These are known as a matched filter processor, a dwell gate processor, a split gate processor, a single edge processor, and a dual edge processor. Each of these processes/processors attempts to overcome noise, multipath distortion, and possible transmit beam asymmetry. Multipath distortion causes the received waveform to be distorted in shape (amplitude versus time graph) when compared to the ideal. Under certain circumstances, the distorted TO received waveform can be a mirror image of the FRO received waveform in time when their respective amplitude versus time graphs are compared. Such disadvantages form part of the system which is taught in "Time Reference Microwave Landing System Multipath Control Techniques", by R. J. Kelly, Journal of the Institute of Navigation, Vol. 23, No. 1. This invention overcomes this multipath effect problem as well as optimizing the signal in a noisy environment.
The first type, a matched filter processor, requires knowledge of an apriori received TO waveform and an apriori received FRO waveform. In case of a symmetric antenna transmit beam (and symmetric antenna received pattern) knowledge of only one is sufficient. In the event of multipath distortion of the received waveform, or if assumed apriori data is incorrect, this filter exhibits poor performance. This is due to the fact that basic apriori assumptions are no longer valid and thus increasing error results. This is documented in the paper given by R. J. Kelly and E. F. C. La Berge, "Comparison Study of MLS Airborne Signal Processing Techniques", 1978 I.E.E.E. document 78CH1336-7 NAECON.
The second type, a dwell gate processor, sets a threshold at a predetermined level below the peak value of the received TO and FRO waveforms. This process determines the time at which the received amplitude rises through the threshold, and the time at which the received amplitude fails through the the threshold. The midpoint between the rise threshold time and the fall threshold time is the reference time for the received waveform. This is done for the TO and FRO received waveforms. The measurement sought is the time between these time reference points. This process does not exhibit optimum performance in either a white noise or multipath noise environment.
The third type, a split gate process, finds the centroid of the received waveform by taking the difference of sums of a number of amplitudes on each side of a sampling point. The computation is performed about a sampling point which is shifted in the direction toward making the difference zero. Typically there are two sampling points. One is the peak amplitude, the other is variable. Linear interpolation is used to find the point at which the difference is zero. This process is superior to the dwell gate but inferior to the matched filter under white noise conditions, and inferior to the single edge processor under multipath interference conditions.
The fourth type, a single edge processor, essentially compares the change in slope of the received waveform with the change in slope of an apriori waveform. The assumption in this process is that only one edge of the received waveform is distorted and so the process is performed on the edge assumed to be undistorted. Also this process is intended to function in conjunction with the dwell gate processor. In general, this processor has superior multipath performance but poor noise performance when compared with the other methods, and exhibits poor performance if beam amplitude is too low or if both edges are distorted.
The fifth type, a dual edge processor, is a combination of two single edge processors. This was motivated by fact that both edges of the beam may be distorted and that an unbiased estimator such as a dwell gate or split gate processor would be required. The functioning is similar to the single edge processor in that both compare the change in slope of the received waveform with the change in slope of an apriori waveform. This is done for both edges and an average is taken. The multipath performance of this processor can be superior to that of the single edge processor. Again noise performance is poor compared with other methods.
TRSB-MLS systems are also taught in U.S. Pat. Nos. 4,019,184, Dorey, Apr. 19, 1977 and 4,017,862, Wild, Apr. 12, 1977. The Wild patent teaches the process of determining the time delay between the TO and FRO waveforms by cross-correlating two wavetrains produced by consecutive TO and FRO excitation.